1. Field of the Invention
The invention relates to a simplified method for the preparation of highly effective polyurethanes suitable as thickeners for aqueous systems, the resulting polyurethanes and their use for thickening aqueous compositions.
2. Description of the Prior Art
Polyurethane-based thickeners for aqueous systems are described in numerous publications (cf., for example, DE-OS 1,444,243, DE-OS 3,630,319, EP-A 0,031,777, EP-A 0,307,775, EP-A 0,495,373, U.S. Pat. No. 4,079,028, U.S. Pat. No. 4,155,892, U.S. Pat. No. 4,499,233 or U.S. Pat. No. 5,023,309).
The prior art thickeners have in common the simultaneous presence of (i) hydrophilic segments in a quantity of at least 50 wt. %; (ii) hydrophobic segments in a quantity of at most 10 wt. % and (iii) urethane groups. The "hydrophilic segments" in these thickeners are primarily polyether chains having at least 5 alkylene oxide units as chain segments in which at least 60 mole % of these units are ethylene oxide units. "Hydrophobic segments" are primarily hydrocarbon segments having at least 6 carbon atoms.
As shown by the literature cited and the work reproducing it, thickeners having a satisfactory or good thickening action may only be obtained when the following conditions are adhered to.
1) The hydrophilic, optionally previously lengthened, polyether segments must have a long chain length of at least 6,000, preferably 10,000 molecular weight units. PA1 2) They must be built up almost entirely from ethylene oxide. PA1 3) The hydrophobic segments must possess at least 12 carbon atoms and be joined at least at two chain ends. PA1 4) Hydrophilic and hydrophobic constituents must be present in a weight ratio of 92 to 97% and 3 to 8%, respectively. PA1 a) a polyether component having an OH number of 10 to 30 mg KOH/g and obtained by the alkoxylation of a mixture of PA1 b) a polyisocyanate component containing at least one organic diisocyanate, optionally in admixture with up to 20 mole %, based on the total moles of component b), of higher than difunctional polyisocyanates.
The thickeners should also have as low an inherent viscosity as possible (for example, 10-50 Pa.s/23.degree. C.), so that they can be processed (measured out) without difficulty, optionally in the form of solutions of the highest possible concentration. This requirement rules out, for example, the preparation of long hydrophilic segments by the chain-lengthening reaction of comparatively low-molecular weight polyether diols with diisocyanates, because as the concentration of urethane groups increases, an undesirable increase in viscosity also occurs.
The synthesis of hydrophilic/hydrophobic polyurethanes which are suitable for the preparation of thickeners and which satisfy the above-mentioned requirements can be carried out by various methods known in the literature. In one method polyethylene oxide-based polyethers having a molecular weight of 10,000 to 20,000 and having at least two hydroxyl end groups are reacted with the equivalent quantity of an aliphatic monoisocyanate (C.sub.12 -C.sub.24).
Although this synthesis appears to be relatively simple, it nevertheless has several disadvantages. It requires polyethers having molecular weights which in the conventional industrial production of polyethers can only be reproducibly obtained at great expense. To produce these polyethers generally an alcoholate mixture is initially prepared from ethylene glycol or from an oligomer (di-, tri- or tetraethylene glycol) by the addition of a concentrated sodium hydroxide or potassium hydroxide solution followed by careful dehydration. Then at 90.degree.-150.degree. C. ethylene oxide is added and polymerized under moderate excess pressure.
In order to ensure that during the preparation of these polyethers the polymerization in the final stage still proceeds at an economically justifiable rate, a "covering" of the OH groups with more than 20% alkali, preferably of more than 50% alkali is necessary. This amounts to the use of more than 20 mole %, preferably more than 50 mole %, of alkali hydroxide per mole of hydroxyl groups. Such a high covering leads to the onset of a reaction, in which only low molecular weight diols used as starters are present, to form heterogeneous mixtures which are difficult to stir and which only contain 1 to 2 wt. % of the desired high molecular weight (MW) polyether alcohols aimed for.
Due to these enormous differences in volume during the course of the reaction and to the problems connected therewith regarding the dissipation of the heat of reaction, in practice the preparation of the high molecular weight polyether alcohols is carried out in several stages. Thus, e.g., in a first step with low covering of alkali (for example, 5-10%) a polyethylene oxide having an average MW of 400 to 800 is produced. In a second step with a higher covering of alkali (10-20%) a polyether having a MW of 2,000-4,000 is produced. Finally, in a third step with a covering of 20-60% alkali the desired high molecular weight polyether is produced. The same procedure is necessary when higher functional starters (glycerol, pentaerythritol, sorbitol) are used. This procedure necessarily results in a relatively high expenditure with regard to reaction time, repeated dehydration and analytical technical support and consequently high costs.
The long reaction times and repeated interruptions favor secondary reactions (for example, due to access of air), which can lead to nonfunctional end groups (for example, vinyl groups or carbonyl groups).
The monoisocyanates used for the introduction of the hydrophobic groups are expensive specialized products having a limited market.
The polyether must be absolutely free of water because otherwise the monoisocyanates form long-chain ureas, which lead to turbidity and precipitation in the aqueous systems to be thickened.
In a second method monofunctional polyether alcohols having a molecular weight of 5000 to 10,000 are prepared by the ethoxylation of fatty alcohols Or alkylphenols having 12 to 24 carbon atoms and then are reacted with diisocyanates.
The previously mentioned problems associated with the preparation of the polyethers also exist here. The alcoholate mixture used as the starter corresponds to about 3 to 6 wt. % of the quantity of polyether alcohol obtained and the covering of catalyst necessary for the final stage of the reaction cannot be supplied at the beginning. In the preparation of alcoholate from the alcohol/sodium hydroxide solution mixture by dehydration, the outcome is troublesome steam distillation of the starter and crystallization out (in the case of solid compounds) in the column and in the cooling apparatus of the distillation device.
In a third method an NCO prepolymer is initially synthesized from diisocyanates and the polyether alcohols prepared by the method described under 1) and is subsequently reacted with fatty alcohols or alkylphenols having 12 to 24 carbon atoms.
In this method the use of expensive monoisocyanates is avoided. But the problems relating to the preparation of polyethers cannot be avoided. In addition, the synthesis can lead only to a regulated structure only if diisocyanates having NCO groups of different reactivities are used (for example, 2,4-tolylene diisocyanate). Otherwise in the first stage a considerable proportion of the diisocyanate remains unreacted and in the second stage forms water-insoluble diurethanes with the monoalcohols. This difficulty is not completely avoided even when using diisocyanates having isocyanate groups of different reactivities.
Polyurethane-based thickeners are suitable as auxiliary substances for adjusting the rheological properties of aqueous systems such as automotive and industrial coatings, finishes and paints, printing inks and textile dyes, pigment printing pastes, pharmaceutical and cosmetic preparations, formulations for plant protection and filler dispersions. Although the known polyurethane thickeners may be used in many applications, their thickening action which is often too low for many application and, as previously described, they may only be obtained in an involved and uneconomic manner.
Therefore, an object of the present invention is to provide a simpler method for the preparation of polyurethanes which are effective as thickeners for aqueous systems and are at least equal to prior art products with regard to their thickening action, particularly under low shear conditions. A good thickening action under low shear forces is particularly necessary in order to have a favorable influence on the flow of aqueous latex paints.
This object may be achieved in accordance with the process of the present invention described in more detail below.